GaN, which is a compound semiconductor with a wide band gap, has promise as a material for a device capable of fast operation with a high breakdown voltage because of its physical property and is expected to be applied, in particular, to a power supply apparatus which operates with a high breakdown voltage and a large current. Further, a Schottky barrier diode (SBD) is superior to a pn diode in terms of faster responsiveness and lower loss. Therefore, an SBD using GaN (GaN-based SBD) is expected as a low consumption power supply apparatus of the next generation.
To decrease a loss of a GaN-based SBD, it is important to decrease the on-voltage of the SBD. Further, to decrease the on-voltage, it is effective to use a metal with low work function for the anode electrode. This is because a Schottky barrier height at an interface between a metal and a compound semiconductor is decreased to lower a forward voltage. On the other hand, a metal with low work function thins the depletion layer occurring when a reverse bias is applied, resulting in difficulty in obtaining a sufficient off-breakdown voltage. As described above, it is difficult to balance a low on-voltage with a high off-breakdown voltage.
Relating to the balance between a low on-voltage and a high off-breakdown voltage, a structure of an anode electrode in which a metal with a small Schottky barrier height is used with a metal with a large Schottky barrier height formed around the former metal is proposed. To form the anode electrode, however, it is necessary to form one of the metals and then form and pattern the other metal. Therefore, a chemical or the like used for cleaning of a surface of a compound semiconductor is limited, and it is difficult to perform sufficient cleaning on an interface between a metal and a compound semiconductor. Consequently, a Schottky characteristic decreases and/or a yield of the device decreases.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2004-31896